The two most common choices for overrunning clutches in high speed environments, such as the gear sets of automatic transmissions, are those with sliding wedging elements, generally referred to as sprags, and those with rolling wedging elements, usually cylindrical rollers. In sprag clutches, the wedging elements are typically dumbbell shaped with sloped end surfaces that jam between two confronting cylindrical race surfaces to lock up in one relative direction, while allowing overrun in the other. Since they do not spin, the sprags are not especially speed sensitive. However, sprags are not as strong as cylindrical rollers, so a large number of them must be used, and they are expensive to manufacture, creating a large cost penalty relative to roller clutches. The rollers in a roller clutch are located in wedging pockets that are formed by a cylindrical surface on one race, usually called the pathway, which confronts a series of sloped cam ramps formed on the other race, usually called the cam race. The rollers are spring urged toward the narrow end of the pocket to a ready position, that is, a position in which they are in continual contact with both the pathway and their respective cam ramps. From ready position, the rollers can quickly jam between and lock the races together if they attempt to change their direction of relative rotation. The drawback of roller clutches is that they are speed sensitive, a problem which manifests itself in two different ways, depending on whether the clutch has an outer or an inner cam race.
In the case of an outer cam roller clutch in which the outer cam race also sees high absolute rotational speeds, centrifugal force can throw the rollers out, away from the pathway and away from ready position. The force of the outwardly thrown rollers can actually overcome the force of the energizing springs that are supposed to keep them in ready position at the narrow end of the wedging pockets, a problem referred to as roller drift. A known solution is shown in the U.S. Pat. No. 4,549,638 to Johnston, assigned to the assignee of the present invention. At least some of the energizing springs are mounted to tracked weights which, at high speeds, slide opposite to the direction of roller drift and compress the springs more, thereby pushing harder on the rollers to keep them in ready position. If the outer cam race turns slowly, so that there is no significant roller drift counteracting the energizing spring, the energizing spring force alone can cause roller-pathway traction, and consequent roller spin and wear on the cam ramps. Roller spin is often a worse problem in the case of inner cam clutches, however, as is explained more fully next.
In an inner cam clutch where the inner cam race sees high absolute speeds during overrun, the rollers are thrown outwardly, but are thereby moved more strongly into the pathway, rather than away from it. The traction of the pathway on the rollers can therefore cause them to spin, often very rapidly. Two different mechanisms may bring the rapidly spinning rollers into contact with the cam ramps. If the rollers are stable at overrun, then their own energizing springs keep them continually in contact with the cam ramps, at the ready position. If the rollers are unstable during overrun, twisting, skewing and moving back and forth intermittently in their wedging pockets, they may be forced into the cam ramps even more strongly. In either event, the spinning rollers have the potential to dig into and locally wear the cam ramps, which can disrupt the cam ramp strut angle. In the case where the inner cam race does not see high absolute speeds, centrifugal force would not contribute appreciably to pathway traction on the rollers. However, the force with which the energizing springs keep the rollers in continual contact with the rapidly relatively moving pathway can still lead to rapid roller spin and wear.
The prior art recognizes the roller spin and wear problem during overrun, in at least one instance. But the solution proposed there may not work well when it is most needed, and may not be needed when it would work well. U.S. Pat. No. 2,044,197 to Barthel proposes biasing the rollers to ready position indirectly, through weighted blocks that are pivoted to the cam race in such a way as to pivot away from the rollers, and reduce the spring pressure, when the cam race is rotating at high speed. In its FIG. 1 embodiment, where the inner race is the cam race and is rotating at high absolute speeds, the spring pressure would in fact be reduced, but centrifugal force would still throw the rollers radially outwardly and into the pathway, causing pathway traction and roller spin. Although the springs would no longer be forcing the spinning rollers into the cam ramps, unstable rollers could still hit the cam ramps, as noted above, and cause wear. In Barthel's FIG. 3 embodiment, where the outer race is the cam race and is rotating at high absolute speeds, the spring pressure would again be reduced, but centrifugal roller drift would already be acting to reduce the spring pressure, anyway. And, in either embodiment, if the cam race were not moving at high absolute speed, there would be no effect on spring pressure at all. Furthermore, the large pivoted weights take up so much room between the races that the energizing springs may be consequently too small to be practical.